The purpose of tank calibration in general is to develop a tank chart for converting heights to volumes. The tank chart, also known as a tank strap chart, converts liquid level readings, in units of length (typically mm or inches), to volume, in units of volume (typically liters or gallons). Tank strap charts are used by gasoline service stations to determine the amount of fuel in a tank. They inform the operator when the tank needs refilling. They are also useful in reconciling dispensing meter amounts against tank readings for tracking tank system integrity or liquid inventory for business purposes. It is necessary that the tank chart be highly accurate for this purpose to minimize discrepancies. Tank strap charts have also found use in determining whether there is a leak in the tank and at what rate liquid is leaking into or out of the tank. This latter use of tank strap charts has taken on increasing importance recently as a result of heightened environmental concerns.
Tank strap charts are typically calibrated by comparing the volume or change in volume per unit of height change in the tank as measured by metering devices and liquid level sensor with estimated volumes or volume changes per unit of height change determined using the tank strap chart. Such techniques use metering devices which are typically integrated with tank dispensers to measure the amount of liquid dispensed into and out of the tank. These measurements are generally considered quite accurate because of the precision of the instruments used in taking these measurements. Liquid level sensors are also used to measure the height of the liquid in the tank. These devices are disposed within the tank, and are also considered quite accurate.
From the height measurements, the estimated volumes or changes in volume can be determined using the tank strap chart. By comparing the "true" volume changes to the estimated volume changes, errors between the data can be determined. These errors are in turn used to refine the tank strap chart. The steps of comparing the volume changes and refining the tank strap chart are repeated until the error between the "true" volume changes and the estimated volume changes is within an acceptable range. Once this is achieved the tank chart his been calibrated.
Known calibration techniques have several drawbacks. One of the problems is that since input data are only available over a limited height range, typically in the middle of the tank (which is where most of the dispensing activity occurs), the chart cannot be accurately extrapolated to the top and the bottom, which are the two regions where the greatest discrepancies often exist and accuracy is needed most. For instance, low level accuracy is needed to determine the amount needed to refill the tank. Low and high level accuracy is needed to verify the amount of a refill.
Another technique generates the tank strap chart from a theoretical model which is based on the dimensions of the tank. Such a technique is disclosed in U.S. Pat. No. 4,977,528. This techniques utilizes a microprocessor which is programmed to determine the dimensions of the tank by reducing errors between the actual volumetric amounts of liquid in the tank derived from the metering equipment and the amounts of liquid calculated based on the assumed dimensions of the tank. However, this technique does, not take into account such factors as the tilt of the tank relative to the gravity field and the position of the sensor in the tank. Both of these factors have an impact on the accuracy of the estimated values especially at the top and bottom of the tank. Furthermore, this technique fails to remove outliers, i.e., data points having a low signal-to-noise ratio, which may result from among other things measurement errors and waves in the tank.
Yet another drawback of available calibrating techniques is that none of them take into account the dynamic condition of the liquid in the tank when the instantaneous Opening and closing height readings are taken.
The present invention is directed at providing a solution to the above mentioned problems.